The present inventions relate generally to semiconductor protection devices, and more specifically to a device for providing series current limiting of sensitive electronics against large currents and voltages during various surge events.
Protection against surge events is an important part of circuit design. The surge events can take the form of either surge currents or voltages and can be caused by lightning, short circuits and various faults in the power systems. The primary protection from these surges is accomplished by circuit breakers, fuses, shunts, and gas discharge tubes (GDT). All of these protection devices have limitations, and electronic circuits still can be damaged by voltages typically in the range of tens to hundreds of volts and currents below one ampere. Thus, a secondary protection is typically accomplished by thyristors. However, the use of thyristors requires a careful design of the coordination between primary and secondary protections.
Even with careful surge coordination, prior art surge protection designs have particular weakness and generally lack reliability. There has been a long-felt need for a simple secondary inline protection device. The primary protection device lets through much smaller amounts of energy, with predictable current and voltage levels. This energy can be blocked by a series secondary protector (in line with the transmission line). Ideally this device should add no more than a small series resistance during regular operation of the circuit.
During a current surge event, the device should block the current above a desired level and be able to withstand the secondary surge voltage. FIG. 1 shows the schematic I-V curve, including all quadrants, for the ideal secondary surge protector operation. Previously in the prior art, transient blocking units were proposed to achieve characteristic performance shown in FIG. 1. A variety of such transient blocking units have been published as U.S. published applications 20080192394, 20060285264, 20060176638, 20060098373, 20060098364, and 20060098363, all of which are hereby incorporated by reference.
FIG. 2 shows such a prior art device made from two MOSFETs as shown (G1, D1 and G2, D2 respectively). Notice that, to operate at small voltages, each MOSFET should possess a small threshold voltage, and therefore a small gate oxide thickness at G1 and G2. In practice, in embodiments designed for handling significant voltage, one cannot apply full drain voltage to the low threshold gate, because the thin gate oxide may be damaged and the device may be destroyed. Therefore, additional circuit elements had to be added in order to overcome the above problems.